As the email system is widely used, incidents of information leakage by email produce serious effects. According to statistics of Japanese Information Processing Development Corporation (for example, fiscal years 2005 to 2007), the leading cause of incidents of information leakage is wrong transmission due to wrong addressing of mail, fax, and email. The leakage incidents caused by wrong transmission of email account for about 5.7% of all leakage incidents.
Most of the incidents of information leakage through email transmission are caused by human errors, such as carelessly setting wrong addresses (destination addresses) of outgoing email. In general, the sender does not notice the careless mistake, and the mistake is recognized when the receiver of the wrong transmission points out the mistake. Therefore, one incident may cause significantly adverse effects. There is a trend of expansion in the use of the email system, and the incident ratio is expected to rise. Therefore, it is more important to prevent wrong transmissions caused by human errors such as careless mistakes.
Conventionally, a mechanism is provided as a prevention measure of wrong transmission of email, in which risk information is presented to the sender before the email transmission to warn the sender to check the address and pay attention. For example, there is a known system, in which a security policy is registered in advance, and when a planned outgoing email violates the security policy, risk information is presented to the sender for warning.
Another example of a conventional method includes a system, in which a white list including reliable addresses registered in advance is prepared, and an address checking process is prevented when the address of an outgoing email is registered in the white list to thereby reduce the load of the sender in the security check process.
Another example of a conventional method includes a system, in which a transmission log of email is stored, an intimacy between a sender and a planned receiver as well as a threshold of availability of transmission according to the intimacy are stored based on the transmission log, words used in the text of an outgoing email are analyzed, the analysis result is determined by the threshold according to the intimacy of the receiver of the planned outgoing email, and the availability of transmission is checked.